Large surface area burns cause loss of the protective skin barrier and dysregulation of antimicrobial defense mechanisms resulting in ideal conditions for colonization of burn wounds and dissemination of bacteria into blood, lung and other tissues. Although topical and systemic antibiotics are routinely used, sepsis is the most common cause of prolonged hospitalization and death in burn patients that survive the initial insult. The frequency and severity of sepsis is exacerbated by the increasing prevalence of antibiotic resistant bacteria. Therefore, there is a need for new treatments to improve resistance to infection in severely burned patients. Our studies show that treatment with the toll-like receptor 4 (TLR4) agonist monophosphoryl lipid A (MPLA) is effective in improving innate host resistance to Pseudomonas aeruginosa burn wound sepsis in mice. MPLA has low toxicity and is currently used widely as a vaccine adjuvant in the clinical setting. However, the mechanisms by which MPLA augments the host response to systemic bacterial infections are not well understood and further work is needed to translate the use of MPLA in high risk patients to the clinical setting. The goal of this project is to identify the cellular and molecular mechanisms by which TLR4-targeted immunomodulators, such as MPLA, improve innate host resistance to infection in high risk subjects. Pre-clinical studies will establish the safety and efficacy of MPLA in large animal models of burn wound infection and post-burn pneumonia. Specific Aim 1: To define the cellular mechanisms by which MPLA augments innate antimicrobial functions. This aim will test the hypothesis that treatment with MPLA will improve the response to infection after burn injury by inducing G-CSF-mediated expansion of the bone marrow neutrophil pool and causing augmentation of neutrophil chemotaxis. Specific aim 2: To define intracellular signaling alterations by which MPLA augments resistance to infection. This aim will test the hypothesis that MPLA treatment will cause alterations in infection-induced signaling resulting in predominance of the protective PI3K and Trif/IRF-3/IFN? pathways. Specific Aim 3: To determine the safety and efficacy of MPLA in an ovine model of burn wound infection and post-burn pneumonia. This aim will test the hypothesis that MPLA will be well tolerated in burned sheep and will improve their response to wound infection and pneumonia. The results gained in the proposed studies will facilitate the translation of TLR4-targeted immunomodulators into the clinical setting and will fill important gaps in knowledge by advancing our understanding of the cellular and molecular mechanisms by which TLR-based immunomodulators improve host resistance to infections.